The present invention is directed to methods and apparatus for recanalizing occlusions, i.e., atherosclerotic plaque build-up, in blood vessels, thereby permitting access to the occlusion by apparatus for resolving and removing the occlusion in the blood vessel in order to improve blood flow therein.
The single largest cause of cardiovascular disease is sclerosisxe2x80x94a build-up of fatty or calcific deposits in the arterial lumen. These deposits can impair, and in severe cases, totally obstruct, i.e., become an occlusion, the flow of blood through the artery. A number of medical devices have been designed to displace, disperse or extract the occlusive deposits. Most of these devices operate over or in conjunction with a guide wire used to navigate the vasculature and traverse the occlusion location. The initial placement of the guide wire can be problematic in cases of total or near-total occlusion. Known techniques for traversing a total occlusion involve forcibly advancing a blunt catheter through the occlusive material (the Dotter technique), or using rotational means (orthogonal displacement of friction).
A more recent development involves using an RF-activated guide wire to electrosurgically recanalize the occlusive material, the details of which are described in U.S. Pat. No. 5,364,393 issued to Auth et al., which is fully incorporated herein by reference for all that it discloses and teaches. As taught in Auth, an electrically conductive guide wire is proximally connected to a radio frequency (RF) generator, which when operated transmits RF energy through the guide wire to a spherical ablation tip.
More particularly, the guide wire is first advanced through the vasculature of a patient, via a guiding catheter or guide sheath, until it reaches the occlusive material. With the spherical ablation tip in contact with the occlusive material, the RF generator is operated and the guide wire tip is advanced through the occlusive material. A therapeutic device used to treat the occlusive disorder is then advanced over the guide wire in accordance with known techniques.
Although the recanalization technique taught by Auth is generally effective, it is desirable to improve the devices and methods used in this approach to provide a more efficient and safe treatment of sclerosis caused by total or near-total occlusions within the arterial vasculature.
In particular, to remove or ablate the occlusive tissue matter quickly and effectively, the electrode tip of the ablative guide wire must be supplied with a potential high enough to ionize or break down the liquid contained in the tissue. This is known as a xe2x80x9cspark erosionxe2x80x9d process. With a monopolar guide wire electrode tip used in conjunction with a dispersive electrode or ground pad located on an external portion of the patient""s body, an ionizing arc from the electrode tip is used to instantaneously convert the occlusive matter into a plasma state, in effect, vaporizing the tissue into particulate matter that is safely absorbed by the blood stream. Once the spark erosion process is initiated, a lower energy potential may be employed to maintain the plasma conversion as the guide wire tip is moved through the occlusive matter.
Towards this end, the RF generator system must provide the guide wire electrode tip with sufficient voltage or potential to initiate the spark erosion process. Typical RF generators, such as those used in electrosurgery or electrophysiology, are capable of generating a high potential, but deliver a constant output level under all load impedance conditions. Within the body, however, the load impedance seen by the guide wire electrode tip may vary greatly, depending upon the relative liquid content of the body tissue it contacts, i.e., the lower the liquid content, the higher the impedance. For example, blood impedance will typically range from 150 to 200 ohm/cm. Healthy vessel wall impedance will typically range from 300 to 400 ohm/cm. Occlusive tissue impedance, on the other hand, depending on the degree of calcification, will normally exceed 600 ohm/cm, ranging from 1000 ohm/cm to as high as 3000 ohm/cm.
Because it is difficult to determine the exact position of the guide wire electrode tip within an occluded vessel, producing a sufficiently high potential to initiate the spark erosion process can be problematic. In particular, when in contact with relatively low impedance blood or healthy vessel wall tissue, a sufficient potential is difficult to achieve without increasing the output power to a level that may cause damage to tissue remote from the surgical site, e.g., in the form of unwanted charring or ablation of healthy tissue. The increased power may also pose risk of a dangerous electrical shock to the attending surgeon, as well as loss of control sensitivity.
U.S. Pat. No. 5,300,068 (xe2x80x9cRosarxe2x80x9d) discloses an RF generator system for selectively providing a train of modulated electrical energy pulses in a modulated continuous wave signal (preferably a cosine squared wave shape) to an electrosurgical electrode disposed on a guide wire, wherein the output impedance of the source of the pulses is continually matched to the load impedance seen by the electrode. In particular, the Rosar generator system measures the relative electrical energy produced by an arc in response to a given electrical pulse, and compares the relative electrical energy to a predetermined value to determine an energy difference. The energy level of a subsequent pulse is then adjusted to reduce the measured difference towards a pre-selected value. According to the Rosar patent, this automatic impedance matching compensates for the changing impedance conditions at the electrode, to ensure an efficient power transfer takes place. In particular, maximum power transfer will occur if the output impedance at the electrode tip is substantially equal to the load impedance of the body tissue (or blood) in contact with the electrode tip.
However, the Rosar generator system is relatively complex and, thus, expensive to implement. Further, because the ablation electrode power output is maximized over all impedance levels, overheating of the electrode in the blood pool or when in contact with healthy vessel wall tissue may result, thus damaging the electrode structure and potentially harming the patient.
Thus, it would be desirable to provide a simplified RF generator system for providing energy pulses to the electrode tip of an ablation guide wire of a voltage potential sufficient to initiate the spark erosion process when in contact with relatively high impedance occlusive tissue, but which will minimize power output when in contact with relatively low impedance blood or healthy vessel wall tissue.
The present invention is directed to improved catheter devices and methods for recanalization of an occluded blood vessel within the vasculature of a patient.
In accordance with one aspect of the invention, centering mechanisms are provided for properly positioning an ablative guide wire tip within an occluded blood vessel for performing a recanalization of the blood vessel.
In a preferred embodiment, a centering catheter for positioning a guide wire in a blood vessel is provided. The centering catheter includes an elongate catheter body having a distal end and an operative lumen. The lumen has a distal end opening, such that a guide wire disposed in the lumen may be advanced beyond the distal end opening. A centering mechanism operable to secure the distal end of the catheter body within a blood vessel is mounted proximate the distal end of the catheter, such that a distal guide wire tip is positioned in the lumen proximate the distal end opening within the blood vessel. It is preferred that the centering mechanism be capable of positioning the guide wire tip both axially and longitudinally within the vessel lumen.
In preferred embodiments, the centering mechanism may be variously constructed to optimize the centering of the guide wire ablation tip depending on the blood vessel geometry at the location of the occlusion. By way of non-limiting examples, in a preferred embodiment best suited for occlusions located in a rectilinear region of a blood vessel, a segmented, inflatable balloon is employed as the centering mechanism. In another preferred embodiment the centering mechanism Comprises a resilient support structure disposed within the catheter body proximal to an inflatable balloon. The resilient support is pre-shaped into a selected complex geometry, such as, e.g., a helix or a bi-planar wave, to best conform the catheter body to wall of the blood vessel. Preferably, the resilient support is composed of a shape memory material, such as, e.g., Nitinol.
A preferred method for recanalization of a blood vessel employing a guide wire centering mechanism includes:
positioning a conductive guide wire having a distal end ablation tip into a blood vessel, such that the ablation tip is adjacent an occlusion to be traversed;
centering the guide wire ablation tip within the blood vessel;
conveying radio frequency (RF) RF energy through the guide wire to the distal ablation tip; and
advancing the energized ablation tip through the occlusion.
In accordance with a further aspect of the invention, a catheter for use as, e.g., a centering catheter is provided wherein air trapped in a distal end inflatable body may be readily purged. In a preferred embodiment, the catheter includes an elongate catheter body having proximal and distal ends. An inflatable body defining an interior region is mounted to the catheter body proximate the distal end of the catheter body. A first lumen extends through the catheter body and has a distal opening in communication with the interior region. A second lumen is disposed (e.g., concentrically) within the first lumen and also has a distal opening in communication with the interior region.
To purge air trapped in the inflatable body, a pressurized fluid medium is introduced through the first lumen into the interior region, forcing the air back out through the second lumen. Preferably, the first and second lumens terminate at opposite ends of the interior region, so that substantially all of the trapped air is pushed out of the interior region.
In accordance with yet another aspect of the invention, methods for manufacturing an insulated, conductive guide wire for use in intravascular medical procedures are provided. A preferred method includes:
placing an insulation tubing over at least a portion of an electrically conductive wire;
stretching the tubing to reduce its thickness to a desired level; and heating the stretched tubing to thereby adhere the tubing wire.
In accordance with a still further aspect of the invention, ablation guide wire tip structures are provided to more efficiently ablate occlusive tissue using a decreased amount of RF power. Towards this end, an ablative guide wire assembly includes an elongate conductive guide wire having an ablation tip formed on its distal end, the ablation tip having a generally non-traumatic structure with a discontinuous feature.
By way of non-limiting example, the non-traumatic tip structure may be spherically shaped, wherein the discontinuous feature comprises one or more protrusion(s) or edge(s) formed on the structure to thereby form high current densities at the discontinuous points. The high current density provides a corresponding high power density, resulting in more efficient tissue ablation.
In accordance with a still further aspect of the invention, a distal portion of the guide wire is shapeable such that the distal portion of the guide wire can better track through, for example, a curved blood vessel lumen.
In accordance with yet a further aspect of the invention, electrical energy pulses are supplied from an RF generator to a monopolar guide wire electrode tip in a continuous wave form, each pulse having an initially high spike, but with the RMS voltage maintained at a relatively low level. In particular, the guide wire ablation system output voltage and impedance are selected in conjunction with the electrode tip geometry (i.e., depending on the particular current density achieved at the electrode tip) to provide for optimal spark erosion when the electrode tip is in contact with high impedance occlusive tissue, while reducing the overall power output when in contact with blood or healthy vessel wall tissue
Other and further objects, features, aspects, and advantages of the present invention will become better understood with the following detailed description of the accompanying drawings.